1. Technical Field
The present invention relates to optical communications systems, and more particularly to an apparatus and a method for using cross-polarization interferometry in a direct-detection, polarization-multiplexing receiver.
2. Description of the Related Art
Coherent light may be transmitted in two orthogonal polarizations, where polarization refers to the orientations and relative phases of the electric and magnetic fields that make up the light. Light can be filtered through a polarizer, so that only those photons which have their electric fields along, for instance, a horizontal plane, may pass through. The result is that signals of two different polarizations (e.g., vertically polarized and horizontally polarized) may be transmitted along the same medium without interfering. The signals can then be split through the use of birefringent materials, which have different indices of refraction for light of different polarizations.
As the demand for data capacity in high-speed optical transmission systems increases, spectral efficiency (SE, the amount of information which can be transmitted over a given bandwidth) is becoming increasingly important. To achieve high SE, polarization multiplexing (PolMux) is a key technology because it can smoothly work with any modulation formats and can easily double the spectral efficiency from the original SE of the modulation format without PolMux. PolMux accomplishes this by transmitting completely separate signals at the same wavelength, each signal having a polarization that is orthogonal to the other.
For PolMux Amplitude Shift Keying (ASK) systems, data is carried by the amplitude of the optical signals on orthogonal polarizations. The signals are combined by a polarization beam combiner (PBC) at the transmitter. At the receiver, the combined optical signals are separated by a polarization beam splitter (PBS) into two orthogonally polarized, optical signals.
An obstacle to this process is the fact that polarization does not remain constant in optical fibers. As a signal travels through the fiber, its polarization rotates Because of this effect, the alignment between the PBS and PBC is not guaranteed. Each output of the PBS would be a combined signal from both inputs of the PBC, such that the original signals cannot be immediately extracted. In addition, a crossing-polarization beating noise will be generated between the two signals. Coherent detection is currently the only option at the receiver side which can minimize the crossing-polarization beating noise.
However, the coherent detection receiver has drawbacks; both frequency offset and phase offset need to be removed by digital processing which requires large power consumption and complicated system design. Coherent detection also requires a spare narrow line-width laser at the receiver as the local oscillator, which can increase both system cost and complexity.
A direct-detection receiver can overcome all of the above issues. There is no frequency offset or phase offset transmitted with the received signal, and a local oscillator laser is not required. However, to realize direct-detection for PolMux-ASK signals, the crossing-polarization beating noise has to be eliminated or avoided before the signal can be correctly detected. The same direct-detection receiver can also work with any other amplitude modulation (AM) systems with PolMux, for example, Quadrature Amplitude Modulation (QAM), On-Off Keying (OOK), etc.